Concrete is a composite construction material composed primarily of the reaction products of hydraulic cement, aggregates, and water. Water is both a reactant for the cement component and is necessary to provide desired flow (e.g., spread and/or slump) characteristics and ensure consolidation of freshly mixed concrete to prevent formation of strength-reducing voids and other defects. Chemical admixtures may be added to freshly mixed concrete to modify characteristics such as rheology (i.e., plastic viscosity and yield stress), water retention, and set time. Although some of the water reacts with the cement component to form crystalline hydration products, a substantial portion remains unreacted and is typically removed from concrete by evaporation. The continued evaporation of water from concrete can pose problems, particularly when applying a floor covering.
A cementitious composition for forming concrete generally refers to a mixture of natural and/or artificial aggregates, such as, for example, sand and either a gravel or a crushed stone, which are held together by a binder of cementitious paste to form a highly durable building material. The paste is typically made up of a hydraulic cement, such as Portland cement, and water and may also contain one or more chemical admixtures as well as supplementary cementing materials, such as, for example, fly ash or ground granulated blast furnace slag cement.
Early cements were based on calcined lime, which is produced by exposing limestone at an elevated temperature, for example, a temperature well in excess of 800° C., in the presence of an oxygen-containing atmosphere to form quick-lime according the reaction in equation (1).CaCO3→CaO+CO2(g)  (1)
Hydraulic limes are derived from calcined limes that have some amount of clay. The clay provides silicon and aluminum that react with the calcium from the limestone to produce cements having complex compounds that hydrate. These compositions even have the ability to harden underwater. Portland cement eventually evolved from these materials.
Most conventional construction cements are hydraulic, many of which are based on Portland cement. Hydraulic cements set and harden after being combined with water, as a result of chemical reactions induced by the water, and demonstrate an improved strength and stability after hardening.
The discontinued use of volatile components in floor covering adhesives for concrete surfaces has created bonding and delamination problems. Concrete contains water for cement hydration as well as water of convenience to facilitate workability and placement. The water is both chemically bound and entrapped in gel and small capillaries comprising about 30-50% of the paste material depending upon maturity. Water in concrete must be consumed, sequestered or evaporated into the atmosphere before a proper, permanent water-based adhesive bond can be assured. Unfortunately, the time necessary to accommodate the requisite drying process is approximately one month per inch of concrete floor depth for standard weight concrete.
Setting and hardening of hydraulic cements is caused by hydration reactions that occur between the compounds that make up the cement and water, which result in the formation of hydrates or hydrate phases. The cementitious composition begins to progressively stiffen leading to the onset of setting, where additional consolidation of the hydration reactants occurs. Hardening follows setting, which is characterized by a steady growth in the compressive strength of the material over a period that can range from a few days in the case of “ultra-rapid-hardening” cements to several years in the case of ordinary cements.
Portland cement consists of five major compounds as well as some additional minor compounds. The major compounds are tricalcium silicate, 3CaO.SiO2; dicalcium silicate, 2CaO.SiO2; tricalcium aluminate, 3CaO.Al2O3; tetracalcium aluminoferrite, 4CaO.Al2O3.Fe2O3; and gypsum, CaSO4.2H2O. The hydration of tricalcium silicate is represented by the reaction according to equation (2).2(3CaO.SiO2)+11H2O→3CaO.2SiO2.8H2O+3Ca(OH)2  (2)
Upon the addition of water, the reaction rapidly progresses to release calcium and hydroxide ions. Once the water solution becomes saturated, the calcium hydroxide begins to precipitate forming a crystalline structure. Calcium silicate hydrate is also simultaneously formed. As the calcium hydroxide precipitates from solution, the tricalcium silicate continues to go into solution to form calcium and hydroxide ions. The reaction is somewhat exothermic involving the evolution of heat as the reaction progresses.
The formation of calcium hydroxide and calcium silicate hydrate provides “seeds” around which calcium silicate hydrate may continue to form. At a certain point, the rate of reaction finally becomes controlled by the rate of diffusion of water molecules through the layer of calcium silicate hydrate that surrounds the unreacted tricalcium silicate, which progressively becomes slower as the layer of calcium silicate hydrate grows larger.
Dicalcium silicate is hydrated to form the same products as tricalcium silicate according to the reaction in equation (3).2(2CaO.SiO2)+9H2O→3CaO.2SiO2.8H2O+Ca(OH)2  (3)However, the hydration of dicalcium silicate occurs much more slowly and is mildly exothermic in comparison to that for tricalcium silicate.
The reactions of the other major components of Portland cement are more complex and beyond the scope of the background discussion given here. However, the hydration of cement is typically characterized by five distinct phases. Phase I is characterized by rapid hydrolysis of the cement compounds and can result in a temperature increase of several degrees over a period lasting on the order of 15 minutes or longer. The evolution of heat begins to dramatically slow in phase II, the dormancy period, which can extend from one to three hours. In phases III and IV, the concrete begins to harden and the evolution of heat begins to increase due primarily to the continued hydration of tricalcium silicate. These phases can encompass a period of up to approximately 32 to 36 hours. Stage V marks a period of continued hydration, but at much lower rates than experienced in the earlier phases, and continues as long as unreacted water and unhydrated silicates remain and can come in contact with one another. Stage V typically continues on the order of days, if not longer.
More commonly, modern-day cements are formulations of hydraulic cement blends. For example, a hydraulic cement, such as, for example, Portland cement, can comprise up to 75% of ground granulated blast furnace slag. The slag results in a reduction in early strength but provides increased sulfate resistance and diminished heat evolution during the stiffening and hardening stages of the concrete.
Blended hydraulic cements can comprise one or more pozzolan materials, which are siliceous or aluminosiliceous materials that demonstrate cementitious properties in the presence of calcium hydroxide. The silicates and even aluminates of a pozzolan reacting with the calcium hydroxide of a cement form secondary cementitious phases (e.g., calcium silicate hydrates having a lower calcium to silicon ratio), which demonstrate gradual strengthening properties that usually begin to be realized after 7 days of curing.
Blended hydraulic cement may comprise up to 40% or more fly ash, which reduces the amount of water that must be blended with the cementitious composition, allowing for an improvement in early strength as the concrete cures. Other examples of pozzolans that can be used in hydraulic cement blends include a highly reactive pozzolan, such as, for example, silica fume and metakaolin, which further increases the rate at which the concrete gains strength resulting in a higher strength concrete. Current practice permits up to 40 percent or higher reduction in the amount of hydraulic cement used in the concrete mix when replaced with a combination of pozzolans that do not significantly reduce the final compressive strength or other performance characteristics of the resulting concrete.
A lightweight coarse aggregate is frequently designed into a concrete mix to reduce building dead load, enable longer spans, provide better seismic benefits, increase fire resistance, and improve sound insulation. This lightweight material commonly comprises expanded shale, clay, pumice, cinders or polystyrene with a density of about ½ or less than that of normal stone coarse aggregate and is capable of producing concrete that weighs from 800 to 1000 pounds less per cubic yard.
In general, the weight reduction in the lightweight aggregate is achieved by creating a highly porous internal structure that can, unfortunately, also absorb up to 30% water. This water is in addition to the normal water of convenience and can impart an additional amount to the concrete mix equal to 2-3 times that which must normally be consumed and evaporated, thereby further increasing the time-to-dry for adhesive or epoxy application. To prevent workability losses due to water absorption during mixing, transport and placement, porous aggregates must be pre-conditioned with water.
Should the concrete be conveyed to the location of placement by a concrete pump, water absorption by the porous aggregates becomes more critical, since the concrete may be subjected to liquid pressure within the pump and attendant line of up to 1000 psi (69 bar), which greatly compresses the air in the pores and causes significant additional water absorption. Such pressure can force water required for workability into the previously unsaturated pores of the lightweight aggregates (i.e., pores which are not filled when subjected to atmospheric pressure but which can be filled at high pressures associated with pumping). Thus, complete saturation of the pores of lightweight aggregates is preferred to prevent workability loss and potential pump line obstructions under these conditions.
Unfortunately, complete saturation is impractical since prolonged soaking in water will not displace air trapped within the capillaries of the lightweight aggregate, so some loss of mix water during conveyance has to be tolerated. Moreover, water instilled into porous aggregates may quickly evaporate in storage, returning the lightweight aggregate largely to its previous dry condition within days. Thus, pre-wetted aggregates must be used almost immediately to capture the desired benefit.
Moreover, even these methods often do not typically result in fully saturated capillaries. Any remaining empty capillaries, when subjected to pump pressures, partially fill with water in response, compressing the air trapped in the capillaries of the lightweight in accord with the Universal Gas Law, thus resulting in the aforementioned workability losses and potential line clogging during pumping. This can have several consequences: additional water must be added to the concrete mix prior to pumping to maintain workability sufficient to facilitate pumping. Thereafter, when the concrete exits the pump and returns to normal atmospheric pressure, the excess water responds to the compressed air within the lightweight aggregates and is partially forced back out into the mix. This, in effect, increases the water-to-cement ratio, excessively diluting the plastic concrete mix and impacting the hardened concrete's permeability.
The cementitious materials in concrete require water, typically known as chemical water or hydration water, to chemically evolve into a hard, crystalline binder. For example, Portland cements generally require up to about 40% of their weight in water in order to promote complete hydration and chemical reaction.
Excess water has conventionally been added to make concrete more plastic allowing it to flow into place. This excess water is known as water of convenience. A small amount of the water does escape as a result of solids settling during the plastic phase, evaporation at the atmospheric interface, and absorption into accepting interface materials. However, much of the water of convenience remains in the concrete during and immediately following hardening. The water of convenience can then escape into the atmosphere following the hardening of the concrete. The water of convenience, depending on, among other things, the water to cementitious ratio, may represent up to about 70% of the total water in the concrete.
The concrete construction and floor-covering industries may incur both construction delays and remedial costs as a result of water vapor emissions and water intrusion from concrete. For example, adhesives and coatings used in the construction of concrete floors are relatively incompatible with moisture that develops at the concrete surface. Moisture may also create an environment for promoting the growth of mold.
Water tightness in concrete structures is a measure of the ability of the hardened concrete to resist the passage of water. Water vapor emission is proportional to the state of relative dryness of the body of the concrete structure. Once isolated from external sources of water, water vapor emissions are derived from the amount of water that is used in excess of that needed to harden the cementitious materials—i.e., the water of convenience. Depending upon the atmospheric temperature and humidity at the surface and the thickness of the concrete, the elimination of excess water through water vapor emissions can take on the order of many months to reach a level that is compatible with the application of a coating or an adhesive.
There is also a possibility that water may develop under the floor due to flooding, water backup, etc. A hardened concrete that resists water vapor permeation is capable of further protecting any coatings that have been applied to the surface of the concrete. There is a need in the art for a concrete that, once it becomes hardened, is substantially resistant to water vapor permeation.
Installation of an impermeable barrier on the surface of the concrete prior to reaching an acceptable level of dryness may result in moisture accumulation, adhesive failure, and a consequential failure of the barrier due to delamination. Premature application of coatings and adhesives increases the risk of failure, while the delay caused by waiting for the concrete to reach an acceptable level of dryness may result in potentially costly and unacceptable construction delays.
The floor covering industry has determined, depending on the type of adhesive or coating used, that a maximum water vapor emission rate of from 3 to 5 pounds of water vapor per 1,000 square feet per 24 hour period (lb/1000 ft2·24 hr) is representative of a state of slab dryness necessary before adhesive may be applied to the concrete floor.
There remains a need in the art for cementitious compositions that reduce the amount of time needed to reach a desired water vapor emission rate in concrete floors enabling a more timely application of coatings and adhesives.
It is known in the art that certain polymers classified as superplasticizers may be included in concrete in order to reduce the amount of water of convenience needed to allow the cementitious mix to more readily flow into place. Certainly, a reduction in the amount of excess water remaining after the concrete hardens should lead to a reduction in the amount of time necessary to reach a desired water vapor emissions rate. However, the use of superplasticizers alone does not address other effects that influence the rate of water vapor emission from the concrete.
There remains a need in the art for cementitious compositions that further reduce the amount of time necessary to reach a desired water vapor emission rate in concrete floors beyond that which is achieved through a reduction in the amount of water required through the use of a superplasticizer additive.
If attainment of a faster drying lightweight concrete is an objective, the usual method of water reduction by utilizing large doses of super-plasticizers (very high range water reducers) is difficult because of the sensitivity of the mix to the loss of the enhanced efficiency water. Furthermore, high doses of super plasticizers tend to impart a thixotropic characteristic exhibited by workability loss if deprived of mixing shear. This loss of mixing shear often occurs during pump hose movement or delay in concrete supply. Because the efficiency of admixture-treated water is improved, loss of water by temporary absorption into the pores of lightweight aggregates during pressurized pumping has both a substantially greater negative impact on workability and a greater negative impact causing potential segregation and bleeding when the admixture-treated water is released from the pores of the aggregates after exiting the pump.
Similarly, the inclusion of silica fume or metakaolin both well-known, highly reactive pozzolans, possess very high surface areas and therefore again require super-plasticizer to reduce water and maintain workability. It also has been found that highly super-plasticized concrete is more difficult to air entrain. Air entrainment is an important feature of lightweight concrete, since it aids in reducing weight and lowers the mortar density thereby attenuating the tendency of the coarse lightweight aggregate particles to float to the surface and hinder finishing operations.
The absorbed water and resulting added mixture water caused by pumping concrete containing porous lightweight aggregates therefore poses difficulties when accelerated drying of the concrete is desired. As a consequence of concrete hydration and lowering of internal vapor pressure in the mortar, the additional water released from the capillaries of the porous aggregates permeates the mortar in the concrete. While this can be beneficial from the standpoint of promoting more complete hydration of the cementitious binder, particularly in lower water-to-cement ratio systems, it can create a prolonged period of relatively high humidity within the concrete, resulting in moist concrete that must dry out before it can be coated or sealed. Such drying is further retarded in humid climates.
The state of dryness within concrete is usually determined by drilling holes to accommodate in-situ humidity probes. When these probes indicate an internal relative humidity (IRH) of 75%, it is presumed to be representative of the future sealed equilibrium moisture condition of the full concrete thickness. Attainment of 75% relative humidity (some floor coverings tolerances may be slightly more or less) ensures that the concrete surface is ready for adhesive application. Experience in the floor covering industry has validated research data which indicates that if internal humidity probes are inserted to a depth of 40% of a concrete structure having one side exposed to the atmosphere (20% if two sides are exposed) in accordance with ASTM F-2170-09, “Standard Method for Determining Humidity in Concrete Floor Slabs Using in-situ Probes”, and the probes indicate an internal relative humidity of 75%, that this is representative of the sealed future equilibrium moisture condition of the full thickness. If the internal relative humidity is higher than 75%, it is assumed the floor will not accept water based glue and will generate sufficient vapor pressure to delaminate impervious coatings. Below that amount, and absent outside moisture influences, it is assume the structure can accept water based glue and not generate sufficient vapor pressure differential to de-bond impervious coatings. Epoxy sealers are also sensitive to water vapor pressure and consequently, encounter similar problems. Premature application of either water-soluble adhesive or epoxy sealer to under-dried concrete can result in moisture accumulation beneath the applied impervious surface and a potential for loss of bond with the epoxy or flooring. There are sealers that can be applied to attenuate the water vapor emission, but they often fail, resulting in loss of space utilization during repair and occasionally creating costly litigation. To reduce the risk of such problems, floors with excessive humidity may require drying times of up to a year or more.
The substitution of the porous lightweight aggregates which absorb water instead of normal aggregates can prolong drying times by months or a year or more. Research has demonstrated that high performance standard weight coarse aggregates concrete (HPC) can dry to satisfactory IRH condition comparatively rapidly. These concretes have water-cementitious ratios (W/Cs) generally below 0.40 and contain fairly large amounts of cement or cement/pozzolans to achieve an internal relative humidity of 75% as ascertained by ASTM F 2170 “Determining Relative Humidity in Concrete Slabs Using in situ probes.” An example the large water difference is shown in Table 1 below.
TABLE 1dry, lbsdry, lbsdry, lbsNormalHPCLightweight HPCCement300400400GGBFS200400400Sand134012741220Stone17501750Lightweight850Water325285325plasticizer10 oz.40 oz.40 oz.W/C0.650.360.41PCF145150.5118.3AE1.30%1.30%5%Total W/C0.700.390.60Aggregate2323151Water
Other research by Suprenant and Malisch (1998) reported that a 4 inch concrete slab made from conventional concrete required 46 days to reach a moisture vapor emission rate (MVER) of 3.0 lb/1000 ft2/24 hours. In 1990 they reported that a lightweight concrete slab made with the same w/cm and cured in the same manner took 183 days to reach the same MVER, a four-fold increase.
The construction industry, therefore, faces a dichotomy. It can address water absorption by the porous aggregate with as much water as needed to ensure pumpability and avoid critical workability loss in the pump line and deal with the consequent prolonged drying time of up to a year or accept the risk of floor failure by using a sealer to isolate the moisture-laden floor from an applied impervious coating or water soluble glue. The concrete construction and floor-covering industries may therefore incur construction delays and/or remedial costs as a result of water vapor emissions and water intrusion from concrete. Moisture may also create an environment for promoting growth of mold.